School welcomes Professor Oskar Vafek
Oskar Vafek is a new professor in the School of Physics and Astronomy's Fine Theoretical Physics Institute. He comes to the University of Minnesota from Florida State University in Tallahassee, where he was director of the condensed matter theory group at the National High Magnetic Field Laboratory. Vafek’s research in condensed matter theory focuses on quantum materials, particularly 2D heterostructures.
In a recent paper, Vafek and his collaborators examined results from a device made up of two layers of graphene, placed on top of one another and twisted at a so-called “magic angle,” which experimentalists have found produces interference patterns when exposed to a high magnetic field. One of these phenomena is the “Hofstadter Butterfly” where the energy bands form a fractal like pattern, first predicted by Douglass Hofstadter over 50 years ago, observed by experimental efforts only in recent years.
Vafek’s postdoc, Xiaoyu Wang, attended a conference and presented a new theory that the experimentalists believe might be extended to explain their observations. The collaboration between the experimentalists and Vafek’s group studied the device exposed to high magnetic field at very cold temperatures and were able to make precise measurements. These were compared to newer theoretical calculations designed to unpick the symmetry breaking phases observed in Magic Angle devices. The result was a paper published in Nature Physics that made two contributions to the theory of Magic Angle devices. The first was observing and explaining predictable Chern Insulator phase in the devices –previously these phases had appeared randomly, but the team was able to correlate them to the insulation figures for the materials. The second was to observe the fractional Quantum Hall effects that only appear when the device was exposed to a strong magnetic field. Vafek and Wang found that the fractional states arise from strained magnetic sub bands with finite bandwidth and irregular geometry. The next phase of this work will be to understand the interplay between the Chern Insulator state and the fractional Quantum Hall state.
Vafek says that there are many exciting experimental results in the field that are still unexplained. “We are trying to solve very difficult problems, experiments give us a clue, they help advance my thinking and we are able to advance the thinking of experimentalists.” The relative accessibility of these types of experiments, which don’t require billion dollar collaborations, has meant that 2D heterostructures have been able to reveal profound new physics of quantum matter. Physicists are searching for answers in the physics of quantum materials to help them create new and more powerful computers as well as low temperature superconductors that could revolutionize the way energy is transported and stored.
Vafek has one graduate student planning to join him at UMN in the spring, as well as postdoc Jun Ho Son, whose position is funded by the Simons Foundation. Vafek is looking for graduate students at UMN to help him carry out his future research.